1. Field of the Invention
The present invention relates to a color processing apparatus and color processing method and, more particularly, to a color processing apparatus and color processing method, which generate a color space which matches human appearance.
2. Description of the Related Art
As a color system required to quantitatively express colors, various color spaces are available. For example, a CIELAB space and CIELUV space specified by CIE (International Commission on Illumination), a JCh space in CIECAM02, and the like are typical examples. However, these color spaces are non-uniform color spaces for human appearance.
FIG. 1 shows a state in which MacAdam's color discrimination thresholds (D. L. MacAdam “Visual sensitivities to color differences in daylight” Journal of the Optical Society of America, Vol. 32, No. 5, pp. 247-274, May 1942) respectively for 25 colors are plotted on a CIELAB space. Note that in FIG. 1, MacAdam's color discrimination thresholds (to be referred to as MacAdam ellipses hereinafter) are enlarged to 10×, and only pieces of chromaticity information are plotted on an a*b* plane for the sake of simplicity.
Each elliptic graphic shown in FIG. 1 indicates a range in which one recognizes the same color. For low-saturation colors, areas of the graphics are relatively small. For high-saturation colors (especially for blue and green), areas of the graphics are very large. That is, one can discriminate low-saturation colors even when distances in the color space are small, but cannot discriminate high-saturation blue and green colors even when distances in the color space are large. The CIELAB space does to match human appearance.
Japanese Patent Laid-Open No. 2002-150277 (literature 1) describes a method of generating a color space which matches human appearance. The method of literature 1 divides a color space which is not uniform for human appearance such as a CIELUV space (to be referred to as a non-uniform color appearance space hereinafter) into small regions such as tetrahedrons (triangles in case of a two-dimensional space). Then, vertex positions of respective tetrahedrons are optimized so that uniform color difference ellipse data such as MacAdam color discrimination thresholds are expressed as perfect circles, thereby correcting the CIELUV space to a color space which is uniform to human appearance (to be referred to as a uniform color appearance space hereinafter).
However, the method of patent literature 1 suffers a problem of locally abrupt color changes which may readily occur, since it decides moving positions of vertices of tetrahedrons obtained by dividing the non-uniform color appearance space by optimization. Literature 1 describes a method of keeping continuity and a method of preventing inversions among neighboring tetrahedrons in optimization of moving positions of vertices of tetrahedrons. However, since optimization is done for each tetrahedron, abrupt changes readily occur in color areas without any uniform color difference ellipse data or color areas in which a plurality of uniform color difference ellipse data are very closer to each other. Furthermore, generation of a uniform color appearance space by the invention of literature 1 requires a very long processing time.